The sensitive layer of this type of sensor generally comprises a semiconductor oxide, the nature and operating temperature of which are chosen according to the molecules to be detected. The resistivity of the sensitive layer varies depending on the concentration of molecules adsorbed by the oxide layer.
FIGS. 1A and 1B schematically illustrate a top view and a cross-section along an axis AA of a conventional heated sensitive layer sensor, such as described, for example, in papers [I Dewa Putu Hermida et al., “Development of Co Gas Sensing Based SnO2 Thin film”, International Journal of Engineering & Technology IJET-IJENS Vol: 13 No: 01] and [George F. Fine et al., “Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring”, Sensors 2010, 10, 5469-5502].
The sensor comprises an insulating substrate 10 bearing on its upper surface a sensitive layer 12, based on semiconductor oxide. Two complementary electrodes in the form of conductive tracks E1 and E2, are disposed in electrical contact with the sensitive layer 12. The electrodes E1 and E2 are configured to measure changes in resistivity of the zones of the sensitive layer located between the electrodes. To improve the sensitivity, it is desired to increase the adjacent lengths of the two electrodes. For this purpose, the electrode tracks are often formed as interdigitated combs, as shown.
Each electrode includes a contact terminal placed at the periphery of the sensor, outside the active area of the sensitive layer. The terminal of the electrode E1 is set to a ground voltage (0 V) and the terminal of the electrode E2 is set to a measurement voltage M, of the order of 1 V. The sensor is operated by measuring the current flowing between the terminals of the two electrodes.
For heating the sensitive layer 12, a resistive track 14 is provided, which is electrically isolated from the sensitive layer, and placed for example on the bottom surface of the substrate 10. The track 14 is configured to uniformly heat the active area of the sensitive layer, i.e. the areas between the facing fingers of the electrodes. The track 14 is often in the form of a serpentine, as shown. It is supplied between ground (0 V) and a voltage Vh regulated to achieve the desired temperature.
The structure of a heated sensitive layer sensor and the materials used therein are particularly well suited for manufacturing using integrated circuit technologies. The sensor dimensions are then so small that the sensitive layer can be heated up to 350° C. with a power consumption of only 30 mW. It has nevertheless been noted that the sensitive layers of such sensors age faster than in sensors made in other technologies.